FIG. 1 illustrates a block diagram of a parallel optical transceiver module 2 currently used in optical communications, which has multiple transmit and receive channels. The optical transceiver module 2 includes an optical transmitter 3 and an optical receiver 4. The optical transmitter 3 and the optical receiver 4 are controlled by a transceiver controller 6, which is typically an integrated circuit (IC) that performs various algorithms in the optical transceiver module 2.
The optical transmitter 3 includes a laser driver 11 and a plurality of laser diodes 12. The laser driver 11 receives an electrical data signal, Data In, and outputs electrical signals to the laser diodes 12 to modulate them. When the laser diodes 12 are modulated, they output optical data signals having power levels that correspond to logic 1 bits and logic 0 bits. An optics system (not shown) of the transceiver module 2 focuses the optical data signals produced by the laser diodes 12 into the ends of respective transmit optical fibers (not shown) held within a connector (not shown) that mates with the optical transceiver module 2.
Typically, a plurality of monitor photodiodes 14 monitor the output power levels of the respective laser diodes 12 and produce respective electrical feedback signals that are fed back to an average output power monitoring circuit 15. The average output power monitoring circuit 15 typically includes one or more amplifiers (not shown) that detect and amplify the electrical signals produced by the monitor photodiodes 14 to produce respective analog voltage signals, which are then input to the transceiver controller 6. The transceiver controller 6 includes analog-to-digital circuitry (ADC) (not shown) that converts the analog voltage signals into digital voltage signals suitable for being processed by the digital logic of the controller 6. The controller 6 performs an average output power monitoring algorithm that processes the respective digital voltage signals to obtain respective average output power levels for the respective laser diodes 12. The controller 6 then outputs control signals to the laser driver 11 that cause the laser driver 11 to adjust the bias and/or modulation current signals output to the respective laser diodes 12 such that the average output power levels of the laser diodes are maintained at relatively constant levels.
The optical receiver 4 includes a plurality of receive photodiodes 21 that receive incoming optical signals output from the ends of respective receive optical fibers (not shown) held in the connector. The optics system (not shown) of the transceiver module 2 mentioned above focuses the light output from the ends of the receive optical fibers onto the respective receive photodiodes 21. The receive photodiodes 21 convert the incoming optical signals into analog electrical current signals. The analog electrical current signals are then received by a received power monitoring circuit 25 of the optical receiver 4. The received power monitoring circuit 25 includes one or more amplifiers (not shown) that detect the analog electrical current signals produced by the receive photodiodes 21 and produce corresponding amplified analog voltage signals. An ADC of the controller 6 receives the amplified analog voltage signals and converts them into digital voltage signals suitable for being processed by the digital logic of the controller 6. The controller 6 performs a received power monitoring algorithm that processes the digital voltage signals to determine the received power levels.
FIG. 2 illustrates a block diagram of a portion 25a of the received power monitoring circuit 25 shown in FIG. 1 for processing the analog electrical current signals produced by one of the receive photodiodes 21. The portion 25a of the received power monitoring circuit 25 includes an amplifier 35, which is typically a current amplifier integrated in the high-speed Transimpedance Amplifier (TIA), and a resistor, RL, 36, which is tied to ground, GND. The TIA 35 is a high-speed amplifier capable of detecting the high-speed electrical current signals produced by the receive photodiode 21 (FIG. 1). The TIA 35 produces an analog current signal, IRSSI, which passes through RL 36 causing an analog voltage signal, VMON, to be provided at the output of the received power monitoring circuit 25a. The analog voltage signal VMON is then sent to an input of the ADC 37 of the controller 6, which converts the analog voltage signal VMON into a multi-bit digital voltage signal. The controller 6 then processes this multi-bit digital voltage signal to determine the corresponding received power.
With reference to FIGS. 1 and 2, an accepted practice is to select the value of RL 36 such that the maximum IRSSI does not result in the maximum input range of the ADC (not shown) being exceeded, taking into account process spread for the gain of the TIA 35, the responsiveness of the photodiode 21 (FIG. 1), and optical coupling loss from the optical input (not shown) of the receiver 4 (FIG. 1) to the active area of the photodiode 21.
Optical transceivers are currently required to monitor and report the received power value. Typically, network equipment of the optical communications network polls the optical transceivers to obtain this information. Often, the network equipment causes the information to be displayed on a display device so that it can be viewed by a network operator. The information is sometimes used to determine whether a degradation of operating conditions has been detected, thereby allowing corrective actions to be taken, either before or after data has been lost. For example, a change in the received power may be an indication of a suspicious optical link, such as one having worn out connectors, strained optical fibers, etc. The network may then take actions to preserve the integrity of the data, such as, for example, routing the data to another link in the network.
Although FIG. 1 illustrates a parallel optical transceiver module, the description provided herein also applies to serial (i.e., non-parallel) transceivers, receivers and transmitters. Many equipment vendors now require that small form factor (SFF) optical transceiver modules monitor the received power with accuracy as high as 2 decibels (dB) over a range of power levels from a maximum allowed received power level corresponding to a power overload down to a minimum required power level corresponding to the minimum power level needed for the optical receiver to operate. For long-range optical links that use single-mode fiber with speeds of, for example, 10 gigabits per second (Gb/s), the range of received power levels that need to be monitored is from about +0.5 decibel-meters (dBm) down to about −18 dBm, with an accuracy that is better than 2 dB. The accuracy with which the received power can be monitored depends on the accuracy of the received power monitoring circuit and on the bit resolution of the ADC. In many cases, accommodating both the maximum input range in the ADC and the required power monitoring accuracy down to the minimum required received level can be a difficult or unachievable goal.
Accordingly, a need exists for a method and an apparatus for monitoring the received power in an optical receiver with high accuracy over a broad range of received power levels.